1. Technical Field
The present invention relates to a lighting device, and particularly relates to a lighting device comprising light emitting diodes (LEDs) with phosphor layers.
2. Description of the Prior Art
In recent years, blue LEDs are often combined with fluorescent materials to produce lighting devices which can emit white light. Such lighting devices are often used as backlight sources of LCD monitors, traffic signals, illuminated switches, or indicators. Moreover, since the LEDs taken as light sources in the lighting devices only require a low current to operate, the energy consumed by these lighting devices can be considerably reduced in comparison with conventional incandescent lamps or fluorescent lamps. In addition, lighting devices using LEDs as light sources can have a longer service life than that of conventional incandescent lamps or fluorescent lamps.
FIG. 1 is a schematic diagram showing a conventional lighting device employing a combination of a blue LED and a mixed fluorescent material. A blue LED 102 is disposed on a substrate 101, and an input terminal 105 is connected to the blue LED 102 for providing electric energy to the lighting device. The blue LED 102 is covered with a mixed fluorescent material coating 103 comprising fluorescent materials which can be excited by the blue LED 102 to emit red light and green light. Furthermore, the lighting device is covered with a transparent hemispherical enclosure 104 to protect the blue LED 102 and the mixed fluorescent material coating 103 therein from the influence of moisture. However, for the lighting device of such design, the green light emitted by the mixed fluorescent material coating 103 due to excitation by the blue LED 102 is reabsorbed by the material capable of emitting red fluorescent light, which causes the luminescence efficiency (i.e. lumens per watt, lumens/W) of green light to decrease, and thus the luminescence efficiency of the entire lighting device is affected.
For the problem that the green light is absorbed by the material capable of emitting red fluorescent light, in U.S. Pat. No. 7,250,715, one blue LED is covered with separated fluorescent materials which can emit yellow-green fluorescent light and red fluorescent light respectively. As shown in FIG. 2, a blue LED 202 is disposed in a reflection cup 200 having a reflective inner surface and covered with adjacent fluorescent material layers 204 and 206 which are capable of emitting yellow-green fluorescent light and red fluorescent light respectively. By exciting the fluorescent material layers 204 and 206 to respectively emit yellow-green light and red light, the abovementioned problem of green light being absorbed can be solved. However, since the fluorescent material layers 204 and 206 are coated on the same blue LED 202, the luminescent efficiency cannot be optimized due to the mutual influence of the fluorescent material layers 204 and 206. In other words, if a blue LED is chosen for improving the luminescent efficiency of the fluorescent material layer 204, such blue LED does not necessarily result in an optimized luminescent efficiency of the fluorescent material layer 206, and vice versa.